In future wireless communication systems, a high data-rate and reliability are compulsory requirements. However, system performance is often degraded by the fading effect of channels. Diversity is commonly considered to be an effective method of improving system performance There are basically three routes available to realize diversity: space, polarization, and radiation pattern. Given the limited space and low profile structure of modern handheld devices, space and pattern diversity may be difficult to exploit successfully. On the other hand, polarization diversity, basically dual polarization, has been implemented in handheld devices with impressive performance [1-3]. In addition, the idea of three-dimensional (3D) polarization has recently been explored [4,5], with the suggestion that it can help to double or even triple the capacity of wireless systems. However, the most straightforward approach to providing three-dimensional (3D) polarization, namely the use of cubic structures, may not be practical to integrate with mobile devices easily in view of the relative physical dimensions [5], since they are generally rather flat devices—see also U.S. Pat. No. 7,710,343.
Accordingly, it has been proposed [9] to utilize the low profile characteristic of a half mode substrate integrated waveguide (HMSIW) antenna [6-8; 10, 11] to reduce significantly the thickness of a three-dimensional orthogonally polarized antenna. This low profile design is a good candidate for embedding into most mobile devices. The three radiating elements are closely located and the design has been carefully considered to match the nature of wave propagation in complex environments. Moreover, it is not necessary to insert any balun before connecting to the backend RF circuits. Such an antenna is designed to operate around 3.5 GHz and have an impedance bandwidth of more than 150 MHz, so that the antenna can support 4G wireless networks, such as WiMAX.
The geometry of a proposed three-dimensional orthogonally polarized antenna from [9] is shown in FIG. 1. The three-dimensional antenna has three radiating elements: Ant I and Ant II are basically thick-slot antennas, while Ant III is a HMSIW antenna. Coaxial probes are used for feeding all the three radiating elements. Ant I is responsible for the linear polarization in the x-direction, the polarization of Ant II is in the y-direction, while z-directional polarized radiation is contributed by the HMSIW antenna—Ant III.
The half mode substrate integrated waveguide antenna of FIG. 1 is basically a quarter of a substrate-filled circular parallel-plate waveguide with vertical walls on the two straight edges connecting the two parallel plates. The length of the arc is about half of the wavelength of the resonant frequency of the HMSIW antenna Impedance matching can be achieved by adjusting the location of the feeding probe.
Simulations of such an antenna using CST Microwave Studio are described in [9]. The dimensions of the whole simulated model are 70×70×9 mm (x×y×z), which is based on the size of smart phones in common use. The thickness and dielectric constant of the inserted substrate are 6.4 mm and 2.2 respectively. The size of the proposed overall antenna from [9] is about 38×38×9 mm. Detailed dimensions of the individual antennas (in mm) from [9] are presented in Table 1.
TABLE 1Dimensions of the proposed antenna from [9] in mm Ant I Ant IIAnt IIIwidth3.03.0Radius28.8length23.024.8 Thickness6.4thickness6.46.4 Feed10.0feed14.414.4
FIG. 2 illustrates the S-parameters of the antennas from [9] for the antenna design shown in FIG. 1. It can be seen from FIG. 2 that the two slot antennas, Ant I and Ant II (S11, S22), have a wider impedance bandwidth (|Sii|<−10 dB) of 800 MHz from about 3.1 to 3.9 GHz, compared with the impedance bandwidth of Ant III. The operating frequency band of the whole antenna is therefore determined by the impedance bandwidth of the HMSIW antenna (Ant III). The impedance bandwidth of the HMSIW antenna (S33) is approximately 160 MHz from 3.44 to 3.60 GHz. The isolation between port 1 and port 2 (S21) is about −18 dB and better isolations of −45 dB are observed between port 3 and port 1 (S31) and between port 3 and port 2 (S32).
FIG. 3 illustrates the simulated gain and 3D radiation patterns at 3.5 GHz from [9] for the antenna shown in FIG. 1. In particular, FIG. 3(i) shows the results for Port 1, the x-directional linear polarization, FIG. 3(ii) shows the results for Port 2, the y-directional linear polarization, and FIG. 3(iii) shows the results for Port 3, the z-directional linear polarization. It can be seen that the maximum gain of the two thick-slot radiating elements is about 2.5 dBi and the HMSIW antenna has a higher maximum gain of 3 dBi. Based on the simulated results, three-dimensional orthogonal polarization can be achieved by exciting Ant I, II and III cooperatively. The variation of gain at different angles is less than 3 dB, which is suitable for mobile communications.
In the implementation of [9], two thick-slot antennas are responsible for the two planar polarizations, while the third perpendicular polarization is contributed by an HMSIW antenna. The thickness of the antenna is shrunk by the inherent thin structure of an HMSIW antenna. The simulated performance of such a low profile three-dimensional orthogonal polarization antenna demonstrates that reasonable impedance bandwidth and isolation between ports can be obtained.
Although the antenna configuration of [9] provides significant benefits over known antenna configurations for providing three-dimensional, orthogonal polarization for use in a mobile communication device, especially in allowing a low profile or substantially planar geometry, there continues to be a need to reduce the space occupied by such an antenna configuration, such as for embedding in a compact, mobile, handheld device, and to improve its performance.